The primary objectives of this tudy are to evaluate the safety and effectiveness of tomoxetine hydrochloride in pediatric patients who have been diagnosed with attention deficit-hyperactivity disorder as defined by the DSM-IV. Plasma concentrations of tomoxetine hydrocloride provide a measurement of the presence of the drug in the body. An aim of this study is to compare the steady-state plasma concentrations achieved and pharmacokinetics attained in pediatric patients, to the steady-state plasma concentrations achieved, and pharmacokinetics attained in adult patients from prior pharmacokinetic studies. The information collected in this study will permit an assessment of dosage modification necessary to achieve steady-state plasma concentrations in pediatric patients corresponding to those achieved in adult patients, and will identify a dose range that provides maximum therapeutic benefit in the reduction of symptoms of attention deficit-hyperactivity disorder. Preliminary findings include robust effectiveness of tomoxetine for attention deficit-hyperactivity disorder symptoms in children, as well as reassuring safety and tolerability data. In the 11 week, dose ranging arm of the protocol, the investigators were able to establish an optimal effective dose of ~2 mg/kg. A preliminary analysis of the pharmacokinetic data reveals a close parallel to the phaarmacokinetic profile of tomoxetine in adults. This data has permitted the design of a large, industry sponsored controlled trial of tomoxetine.